I would keep the belt on Rollins and have the Authority give him a sort of "sink or swim" ultimatum where he has to prove that he really is the "Man" they thought he was. Call it tough love or whatever. And, while not totally getting rid of the snarky, smarmy, cheating heel aspect of his character, let him get some legit wins on his own, even if he's cheating, it has to be HIM cheating, not someone else interfering on his behalf. If the crowd is responding to him positively, you can toy with the idea of turning him face - he's proven to himself that he doesn't need the Authority, and now begins to see they were just using him all along. If the crowd doesn't start to like him, then you've at least legitimized your heel champion as a true threat and tough guy to beat for the title.
Have Reigns turn. First, after losing to Wyatt, have him get into a little with Ambrose, but not all the way. Then (as someone smarter than me suggested in the PWD thread) have him go off on the fans for booing him, leading to another confrontation with Ambrose. I wouldn't pull the trigger on a full Reigns/Ambrose feud right away, though. Turn Reigns into a lone wolf type heel who is a silent killer. Have Ambrose hold onto hope that he can get his "brother" back, but realize that it's probably not going to happen.
At the Rumble, heel Reigns and face Ambrose are your last two men standing, and they eliminate each other a la Bret and Luger. They are put in a match at the next PPV to decide who gets the title shot at WM. Before the match, and in light of Rollins beginning to pull away from the Authority because he knows he doesn't need them, HHH and Steph approach Reigns and tell him they like the new side of him and that maybe they misjudged him in the past. Reigns and Ambrose start off with a fairly squarely match, that devolves into Reigns going full heel on Ambrose on the outside, smashing him into the post and slamming him on the announcers' table. Ambrose is having none of it and goes lunatic fringe on him. They both grab chairs and shit and fight into the crowd, resulting in a no contest.
As a result, the main event of WM becomes a Triple Threat Match as the Shield explodes: Rollins (c) vs. Ambrose vs. Reigns. Rollins is still, at least in name, the Authority's guy, but he's wary of them and doesn't fully trust them. The Authority is courting Reigns, either because they want him to replace Rollins or as a contingency. One thing for sure, they don't want Ambrose to win, which, of course, he does, after a bruising triple threat match.
To keep up the story from last year, Ambrose pins Reigns for the win, so for the second straight year, Reigns takes the pinfall to lose the main event, and the champion doesn't actually get pinned. Reigns goes apeshit after and hits a spear on Ambrose before Ambrose can even celebrate. Ambrose is down and Sheamus' music hits. He's going to cash in to close out WM, but before he can, Rollins returns to the ring and takes him out with a flying knee, sending him rolling out of the ring and scurrying away without having cashed in.
Rollins and Ambrose stare at each other as Ambrose slowly gets up. There is no handshake or anything, and Rollins leaves the ring. Ambrose's music hits and he celebrates his first WWE title win.
Semi-related to this storyling: part of the legitimizing of Rollins title run will include a win over Cena either in December or at the Rumble. This will be a part of the larger storyline over Cena not being able to win the title. He can beat Rollins for the US title or in non-title matches, but can't get it done when the WWE title is on the line. This is part of a story where the pressure of tying Ric Flair's record is weighing too heavily on Cena. In a perfect world, this leads to Cena turning and aligning with the Authority to finally get over the hump, but in reality he'll eventually just dig deep, never give up, and overcome.